CDC

An adult has died of the plague in Pueblo, Colo., health officials announced yesterday. The victim, who has yet to be identified, likely contracted the disease from fleas on a dead or dying animal, officials say.

Many people think of the plague as a disease from the Middle Ages: something for history books, not modern-day epidemiology. But in the southwest, at least a couple plague cases still pop up almost every year. There were eight reported cases in Colorado last year, according to the state Department of Health; this is the second case so far in 2015.

Almost all of the cases since 1970 have been in the west, concentrated in southern Colorado, northern New Mexico, and northeastern Arizona:

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"We've had a very wet winter and a very cool summer," Ramona Chisman-Ewing, a spokesperson for the Colorado health department, told Fusion. "It was just a great breeding ground for fleas." She said the recent plague cases have been concentrated in the southwest because "we have a lot of wildlife that attract fleas," such as prairie dogs, and a lot of people spend time out in the wild.

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Between 1900 and 2012, 1,006 confirmed or probable human plague cases occurred in the United States, according to the CDC.

CDC

The disease is transmitted from infected fleas, and carried by rodents. Most recent cases had to do with people coming into contact with dead or dying rodents, such as the Oregon man who tried to remove a mouse from his choking cat's throat. It's treatable with antibiotics, but still has a mortality rate of about 11 percent.

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Plague was first introduced in the U.S. by a rat-infested steamship from Hong Kong to San Francisco in 1899. Local officials in San Francisco wanted to cover up the plague, and responded with a racist quarantine of Chinatown. After the 1906 earthquake devastated the city, rats spread the disease through refugee camps.

Worried about the plague? Be sure to keep away from dead and dying rodents (because that's really hard advice to follow…) and watch your pets as well. "They love to chase prairie dogs, and boy, they bring it back, and a lot of people love to have animals sleep in the same room as them," Chisman-Ewing said. "Don't do that."

Casey Tolan is a National News Reporter for Fusion based in New York City.